You can make use of the child theme concept, if you don't have a child theme activated, check this guide:
After that, that's the file you need to copy in your child theme directory:
You will find that specific line you mentioned:
<div class="entry-content content">
Which you can wrap it between
is_single conditional tag:
Also, check this guide while implementing this conditional tag:
As this is all custom development, regretfully we wont be able to assist further. Custom development is outside the scope of our support. We are happy to provide advice and get you started in the right direction, but you would still be responsible for the implementation. And moreover, this solution may not work after several updates especially if the themes structure changed or overridden.